1. Field of the Invention
The present invention relates to a very thin, high carbon steel wire having a high strength and superior in high-speed strandable ductility, as well as a method of producing the same.
2. Description of the Related Art
A very thin, high carbon steel wire having a diameter of 0.05 to 0.50 and a strength as high as 4200 MPa or more has come to be used as steel cord or as saw wire for cutting a semiconductor. Generally, the very thin, high carbon steel wire is produced by subjecting a steel wire rod of 4.0 to 5.5 mm in diameter which has been subjected, as necessary, to hot rolling and subsequent conditioned cooling to primary wire drawing, subsequent final patenting treatment and further wire drawing.
More particularly, the above patenting treatment involves heating the steel wire rod to a temperature range (750-1100° C.) of A3 point or higher for treatment to γ phase, subsequent quenching and allowing an isothermal transformation to proceed in the temperature range of 550-680° C., to afford a steel wire of a pearlite structure. This steel wire then goes through brass plating as a surface-layer lubricant coating treatment and is made into a very thin, high carbon steel wire by a continuous, final wet lubrication wire drawing with use of dies arranged in multiple stages.
Many of high strength, very thin, high carbon steel wires thus produced are used as stranded two- or five-ply steel cords or saw wires for cutting a semiconductor.
The very thin, high carbon steel wires applied to the aforesaid uses are required to possess such characteristics as 1) being higher in strength, 2) superior in high-speed wire drawability, 3) superior in fatigue characteristic, and 4) superior in torsional deformability (ductility) in the aforesaid stranding work.
Particularly, as the strength becomes higher than 4200 MPa, the ductility of each very thin, high carbon steel wire is deteriorated. Consequently, delamination (longitudinal cracking) becomes easier to occur during torsional deformation of the very thin, high carbon steel wire.
Anti-delamination property of the very thin, high carbon steel wire can be grasped and evaluated in advance by a phenomenon that, upon occurrence of delamination in a twisting test of the very thin, high carbon steel wire, there occurs a sudden drop of torque in a rotational angle-torque chart in the twisting test.
For enhancing the anti-delamination property of the high strength, very thin, high carbon steel wire, various techniques have heretofore been proposed with respect to the steel wire. For example, it has been proposed to control the hardness of the surface layer of the steel wire in accordance with the wire diameter and thereby prevent the occurrence of delamination (see Japanese Unexamined Patent Publication No. 2000-336459).
It has also been proposed to control the difference in tensile strength between the very thin, high carbon steel wire just after wire drawing and the very thin, high carbon steel wire after wire drawing and after subsequent age hardening to a predetermined certain range and thereby preventing the occurrence of delamination (see Japanese Unexamined Patent Publication No. 2002-302736).
Various techniques have been proposed also with respect to wire drawing. For example, there has been proposed a technique in which two separated drawing dies having different bore diameters are used and wire drawing is performed once without giving any drawing force between both drawing dies (see Japanese Unexamined Patent Publication No. Hei 7(1995)-1028).
Further, it has been proposed to effect wire drawing by raw drawing without intermediate heat treatment and setting a die approach angle at a low angle during wire drawing to optimize the reduction of area in each stage (see Japanese Unexamined Patent Publication No. 2001-192772).
However, even by the above conventional proposed techniques, delamination cannot be prevented effectively in the case of very thin, high carbon steel wires having a diameter of 0.05 to 0.50 mm and having a strength as high as 4200 MPa or more. This is presumed to be because a new delamination generating mechanism so far not found out is added in the region of those high strength, very thin, high carbon steel wires in comparison with the conventional lower strength region.